The production of melamine starting from urea is a method that has been known for quite some time, a distinction being drawn between two types of processes: The noncatalytic high-pressure process and the catalytic low-pressure process. The high-pressure process requires pressures of at least 8 MPa, while the catalytic low-pressure process is carried out in a fluidized bed at a pressure of 0.1 to a maximum of 1 MPa with temperatures of at least 380 to 410° C. The carrier gas used for the low-pressure method is either ammonia or a mixture of carbon dioxide and ammonia, the resulting melamine being present in gaseous form after the reaction.

The yield of melamine with this method relative to the quantity of urea used is 90 to 95%. In the literature, the three most frequently used low-pressure methods are known as BASF, Chemie Linz and Stamicarbon processes.
The BASF process is a single-stage reaction method (FIG. 1), where molten urea is reacted in a fluidized bed at a temperature of 395 to 400° C. under nearly atmospheric pressure. In addition to melamine and urea, the resulting reaction gas contains traces of by-products such as melem and melam as well as a reaction gas comprising ammonia and carbon dioxide. The reaction gas mixture that is obtained is then cooled, the removed catalyst and the crystallized by-products are separated and the reaction gas comprising the melamine is fed to a crystallizer. In the crystallizer, the hot gas comprising the melamine is cooled with cold reaction gas (quenching gas), which lowers the temperature of the melamine-containing gas to 190 to 220° C. Under these conditions, melamine desublimes approximately 98% from the reaction gas. After separating the melamine, the remaining gas (recycle gas) is pumped to a urea washing station by means of a recycle gas blower, is cooled and washed in direct contact with the molten urea and then recirculated into the reactor. The temperature of the quenching gas is ˜138° C., so that it is necessary to admix 2.5 to 3.5 kg of quenching gas per kilogram of melamine-containing gas to adjust a temperature of 190 to 220° C. in the crystallization apparatus.
The separation of melamine from melamine-containing gas mixtures is a process that has been well-known for quite some time.
German unexamined patent application 1,204,679 [U.S. Pat. No. 3,321,603], for example, describes the separation of melamine from a gas stream by cooling on cold walls, or by contacting it with a cold inert gas stream, a cold inert fluid or cold inert solid mass.
The commonly used method of separating melamine by contacting it with a cold inert gas stream, however, has the disadvantage that very large quenching gas volumes relative to the reaction gas volume are required for the necessary temperature reduction. The ratio is approximately 4 to 1. Due to the large volume of gas, apparatuses with accordingly large dimensions are required, which is associated with corresponding added financial expenses in the production of melamine and with higher investment expenditures. In addition, the gases leaving the product cyclone must be cleaned and recirculated with the help of the recycle gas blower. Also, the recycle gas is still saturated with melamine and desublimes melamine every time the temperature is lowered further. This type of melamine adhesion also occurs at undesirable sites such as the recycle gas blower, resulting in drastically shortened plant operating life, undesirable production failure and an increased need for maintenance.